A typical on-board evaporative emission control system comprises a vapor collection canister that collects fuel vapor emitted from a tank containing volatile liquid fuel for the engine and a CPS valve for periodically purging collected vapor to an intake manifold of the engine. In a known evaporative system control system, the CPS valve comprises a solenoid that is under the control of a purge control signal generated by a microprocessor-based engine management system. A typical purge control signal is a duty-cycle modulated pulse waveform having a relatively low operating frequency, for example in the 5 Hz to 50 Hz range. The modulation may range from 0% to 100%. This means that for each cycle of the operating frequency, the solenoid is energized for a certain percentage of the time period of the cycle. As this percentage increases, the time for which the solenoid is energized also increases, and therefore so does the purge flow through the valve. Conversely, the purge flow decreases as the percentage decreases.
The response of certain known solenoid-operated purge valves is sufficiently fast that the armature/valve element may follow, at least to some degree, the duty-cycle modulated waveform that is being applied to the solenoid. The pulsating armature/valve element may impact internal stationary valve parts and in doing so may generate audible noise that may be deemed disturbing.
Changes in intake manifold vacuum that occur during normal operation of a vehicle may also act directly on a CPS valve in a way that upsets the intended control strategy unless provisions, such as a vacuum regulator valve for example, are included to take their influence into account. When the CPS valve is closed, manifold vacuum at the valve outlet is applied to the portion of the valve element that is closing the opening bounded by the valve seat. Changing manifold vacuum affects the start-to-flow duty cycle, potentially causing unpredictable flow if the valve element does not have sufficient time to achieve full open condition.
One general objective of the present invention is to provide an improved CPS valve that achieves more predictable purge flow control in spite of influences that tend to impair control accuracy. In furtherance of this general objective, a more specific objective is to endow a CPS valve with a characteristic that is effective over a wide range of intake manifold vacuum levels to consistently cause the actual purge flow to more predictably equate to that intended by the purge control signal irrespective of changing intake manifold vacuum. In accomplishing this objective in the inventive CPS valve, valve operation that is quieter than in certain other CPS valves can be achieved.
From commonly assigned U.S. Pat. No. 5,413,082, inter alia, it is known to incorporate a sonic nozzle function in a CPS valve to reduce the extent to which changing manifold vacuum influences flow through the valve during canister purging. The disclosed embodiment of CPS valve which is the subject of the present invention incorporates a sonic nozzle structure at its outlet. From U.S. Pat. No. 5,373,822, it is known to provide pressure-or force-balancing of the armature/valve element.
One generic aspect of the present invention resides in novel means for the integration of force-balancing and intake manifold vacuum de-sensitizing so that the start-to-flow duty cycle is significantly de-sensitized to changing intake manifold vacuum. The inventive CPS valve therefore exhibits quite consistent opening as its valve element unseats from the valve seat; it also exhibits quite consistent closing as the valve element re-seats on the valve seat. Because the inventive CPS valve achieves these consistencies, which are relatively quite well- defined and predictable, the duration within each duty cycle for which the sonic nozzle structure at the valve outlet functions as a true sonic nozzle is also quite well- defined and predictable, being equal to the duration of the duty cycle less the durations of valve element travel at initial valve unseating and at final valve re-seating where the proximity of the valve element to the valve seat prevents the sonic nozzle structure from operating as a true sonic nozzle, uninfluenced by the extent of flow restriction present between the unseated valve element and the valve seat. The sonic nozzle structure will therefore function as a true sonic nozzle over an entire duty cycle except for these initial unseating and final re-seating transitions. By making the valve element travel during which these transitions occur relatively short, the sonic nozzle structure can function as a true sonic nozzle over a larger portion of a duty cycle. Therefore, the inventive CPS valve can enable the actual mass purge flow that will occur during a duty cycle to be accurately correlated to the purge control duty cycle signal, and hence well-defined and well-predictable.
The inventive valve also possesses other novel features which are of benefit in fabricating the valve. One of these features relates to an especially convenient means for setting the valve seat in proper positional relation to the valve element at time of valve fabrication. Another relates to solenoid stator structure that facilitates incorporation of the force-balancing function. Still other features involve certain constructional details that provide additional distinctive benefits.
The foregoing, and other features, along with various advantages and benefits of the invention, will be seen in the ensuing description and claims which are accompanied by drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.